INDUSTRIAL FIELD OF THE INVENTION
The present invention relates to a novel type of whiskers and a fiber reinforced composite material using the same. More particularly, the present invention relates to potassium hexatitanate whiskers having a tunnel structure and containing aluminum and niobium, both of which are impurities, in such amounts that the Al.sub.2 O.sub.3 /Nb.sub.2 O.sub.5 molar ratio is at least 0.6. The present invention also relates to a composite material comprising said whiskers as a reinforce material and a thermoplastic resin or a light alloy as a matrix.
Whiskers such as silicon carbide whiskers, silicon nitride whiskers, aluminum borate whiskers, and potassium titanate whiskers are known heretofore. However, only potassium titanate whiskers among them are put to practical use as a reinforcing material for general use plastics as well as for engineering plastics. This is ascribed to the fact that whiskers other than potassium titanate whiskers are too expensive. The least expensive potassium titanate whiskers, however, are still expensive as compared with glass fibers and wollastonite which are widely used as reinforcing materials for plastics. The application of the composite materials containing potassium titanate whiskers at present is therefore confined to an extremely narrow field. Accordingly, from the viewpoint of extending the use of the known composite materials and of acquiring the role of a reinforcing material for plastics, it is desirable for production cost of potassium titanate whiskers to be substantially lowered to a price comparable to that of glass fibers, or to develop potassium titanate whiskers having a higher strength without increasing production costs.
With respect to the aim of providing low price potassium titanate whisker reinforced plastics, some developments thereon have been made as described, for example, in Japanese Patent Public Disclosure (Kokai) Nos. 60-51615 and 60-191019. However, there has been made no attempt to provide advanced composite materials having a higher mechanical strength at a low price, by taking advantage of whiskers having an improved strength.
Furthermore, potassium titanate whiskers are now attracting attention as a reinforcing material for light alloys to provide a general use industrial material. This tendency is ascribed to the fact that the reinforcing materials suited for reinforcing alloys, i.e., the ceramic fibers, are quite expensive as compared with potassium titanate whiskers. Hence, the use of a relatively cheap potassium titanate whiskers would provide a low-price composite material having a wide range of application. With respect to the production process, sequeeze-casting method is most favorable from the viewpoint of the performance of the resulting composite material and of production costs. This process is advantageous in that the reaction can be accomplished in a short period of time and carried out with relatively simple manufacturing apparatus. Thus, reinforced composite materials which suffer less problems concerning reaction of the reinforcing material and the light alloy matrix can be obtained by a relatively simple process.
It was reported that potassium titanate whiskers react vigorously with an molten aluminum (see Hideharu Fukunaga, Michifumi Takeichi, and Nobuhiro Ohta, J. Jpn. Soc. Compos. mater, 8(1982), p.66), but this problem was overcome by an improved potassium titanate whisker disclosed in Japanese Patent Public Disclosure (Kokai) No. 1-301516, as potassium hexatitanate whiskers having a tunnel structure and a free potassium content of 5 ppm or less.
The potassium titanate whiskers above were obtained by considerably reducing the content of layered structure potassium titanate included in the potassium hexatitanate whiskers having a tunnel structure. The development of such potassium titanate whiskers has enable their use as a reinforcing material for light alloys with a view to providing general use industrial materials (see K. Suganuma, T. Fujita, K. Niihara, T. Okamoto, and S. Suzuki, J. Mater. Sci. Letters, 8 (1989) p.808).
The metal-based reinforced material using the above potassium titanate whiskers as the reinforcing material exhibited improved mechanical strength as compared with the light alloy matrix composite using the conventional potassium titanate whiskers, however, it still was subjected to the disadvantages inherent to the composite using the conventional potassium titanate whiskers. For example, when a heat treatment was applied to the composite material with an aim to improving, for example, mechanical strength, resistance against stress corrosion cracking, and the like, the potassium titanate whiskers decreased strength and/or exhibited no improvement in the intended properties. Further, Suganuma et al. found magnesium-based very narrow reaction layer below 10 nm, was formed on the surface of potassium titanate whiskers during a production of a composition of potassium titanate whiskers and an aluminum alloy. They pointed out that the strength of the matrix is thereby reduced, because magnesium content in the matrix alloy is reduced. However, they also reported that this reaction layer remained unchanged through the heat treatment process after producing the composite, and hence it was still unclear why the strength of the potassium titanate whisker reinforced light alloy should be reduced by the heat treatment.
As described above, the conventional potassium titanate whisker reinforced metal composites were not favorable for use in an industrial material due to their lack of stability during a heat treatment, and hence the development of a novel and economically advantageous composite material capable of increasing its strength by a heat treatment, i.e., a composite material having thermal stability, was not accomplished.
An object of the present invention is to provide more economically a composite material having improved mechanical strength as compared with conventional types, by improving the mechanical strength of potassium titanate whiskers.